Non-aqueous secondary batteries, which are represented by lithium ion secondary batteries, are widely spread as power sources for portable electronic devices such as notebook-size personal computers, mobile phones, digital cameras and camcorders.
In recent years, outer packagings of non-aqueous secondary batteries have been simplified and lightened with size reduction and weight reduction of portable electronic devices. as outer packaging materials, aluminum cans have been developed in place of stainless cans, and further, aluminum laminated film packages have been developed in place of metallic cans.
However, an aluminum laminated film package is soft, and therefore in a battery having the package as an outer packaging (soft package battery), a gap is easily formed between an electrode and a separator due to external impact, or electrode expansion and shrinkage associated with charge-discharge, as a result of which the cycle life may be reduced.
Techniques for improving adhesion between an electrode and a separator have been proposed in view of solving the above-mentioned problem. As one of the techniques, a separator in which a porous layer containing a polyvinylidene fluoride resin is formed on a polyolefin microporous film is known (see, for example, Patent Literatures 1 to 5). When superimposed on an electrode and heat-pressed, the separator is favorably bound to the electrode with the porous layer interposed therebetween, so that the cycle life of a battery can be improved.
On the other hand, along with densification and capacity enhancement of soft package batteries, separators which improve safety of batteries have been proposed. For example, Patent Literature 6 proposes a separator in which a heat-resistant layer containing inorganic particles and a heat-resistant binder resin is formed on a polyolefin microporous film. Patent Literature 7 proposes a separator in which a porous layer containing a polyvinylidene fluoride resin and inorganic particles is formed on a polyolefin microporous film.